Generally, in an optical system, a pickup emits laser light to a disc, receives light reflected from the disc, and then reads data recorded on the disc. At this time, the pickup converts the reflected light into a radio frequency signal and outputs the converted radio frequency signal. The radio frequency signal output from the pickup is then processed by an appropriate signal processor. As a result, a tracking error signal, etc. is generated.
In a conventional optical system, when a pickup jumps tracks or randomly accesses desired tracks, the pickup spot traverses the tracks. A microcomputer calculates the number of tracks to be traversed and controls the pickup to move to the desired tracks.
FIG. 4 is a block diagram of a traverse servo in the optical disc system in prior art. When a pickup 42 jumps tracks or randomly accesses a desired track, the pickup 42 traverses certain tracks of the disc 41. A number of tracks to be traversed is calculated in a microcomputer 46. The pickup 42 also reads data recorded on a disc 41 and outputs a radio frequency (RF) signal. The RF signal is fed to a RF amplifier 43 and a tracking error (TE) generating circuit 45. The amplified RF signal output from the RF amplifier 43 and the TE signal output from the TE signal generating circuit 45 are fed to a track traverse detecting circuit 44. The track traverse detecting circuit 44 outputs a track traverse signal Cout. The microcomputer 46 receives the track traverse signal Cout and calculates the number of tracks to be traversed. The microcomputer 46 commands to a tracking controller 47 to control an actuator driver 48 in a track jump mode or a random access mode and then the actuator driver 48 drives the pickup 42 to jump the calculated tracks so as to move to the desired track. The number of tracks to be traversed is calculated from the track traverse signal Cout generated in the track traverse detecting circuit 44. Then the microcomputer 46 commands the tracking controller 47 to stop the pickup 42 on the desired track.
The radio frequency (RF) signal amplified by a RF amplifier 43 and tracking error (TE) signal generated by the TE signal generating circuit 45 are used in generating the track traverse signal Cout in the jump mode. The TE signal and the RF signal may be used as the track traverse signal Cout. However, both signals are used in generating the track traverse signal compensating each other because the track traverse signal Cout is generated every time the pickup 42's light spot traverses one track.
FIG. 5 is a block diagram of a conventional RF signal processing circuit in the track traverse signal detecting circuit 44. An envelope detector 51 detects a RF envelope signal from the received RF signal. The RF envelope signal is fed to a wave shaping circuit 52. The wave shaping circuit 52 compares the RF envelope signal with a predetermined constant reference voltage and outputs a shaped RF envelope signal. The shaped RF envelope signal is used for a first track traverse signal since its pulse is generated corresponding to the traversed track. To obtain a precise track traverse signal, the TE signal is used as a second track traverse signal at the same time.
However, when the conventional optical system is used for an optical disc player for playing both compact discs (CDs) or digital video discs (DVDs), since the DVDs have a narrower pitch than the CDs, the amplitude of the RF envelope signal of the DVDs is smaller than that of the RF envelope signal of the CDs.
The different amplitudes of the RF envelope signals of CDs and DVDs cause problems with the conventional optical system where the level of a reference voltage Vref for generating the first track traverse signal is fixed to a predetermined reference level according to the pitch of the CDs. Thus, the first track traverse signal is not accurately generated if DVDs are used. This means that the microcomputer 46 is incapable of accurately counting the number of tracks to be traversed at all times. Therefore, to correct the unreliability of conventional optical systems, a reference voltage Vref capable of being adjusted according to the use of CDs and DVDs is needed track traverse signal free of error.